Six of the planets and three of the dwarf planets are orbited by moons. Furthermore, planetary dust orbits the gas giants. There are many other systems like the Solar System in the universe. Each star has the potential to have a planetary system. There are more than 200 billion stars in the Milky Way galaxy.

Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disk of loose dust, out of which the planets, moons, asteroids, and other Solar System bodies formed.

The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have formed independently and later been captured by their planets. Still others, as the Earth's Moon, may be the result of giant collisions.

Many collisions between bodies have occurred, and have been important to the evolution of the Solar System. The positions of the planets often shifted, and planets have switched places.[2][3] This planetary migration is thought to have been responsible for much of the Solar System's early evolution.

Earth's orbit

The Earth's orbit around the Sun is nearly a perfect circle, but when mapped it is found that the Earth moves around the Sun in a very slightly oval shaped, called an elliptical orbit. The other planets in the Solar System also circle the Sun in slightly elliptical orbits. Mercury has a more elliptical orbit than the others, and some of the smaller objects orbit the Sun in very eccentric orbits.

The eight planets

The planets are the biggest objects that go around Sun. It took people many years of looking carefully through telescopes to find the objects that were farthest away. No one expects to find new planets, but more small objects are found every year. Most of the planets have moons that orbit around them just as the planet orbit the Sun. There are at least 173 of these moons in the solar system.

Dwarf planets

Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided on the definition of a planet, and Pluto did not fit. Instead they defined a new category of dwarf planet, into which Pluto did fit, along with some others. These small planets are sometimes called plutinos.

The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth.

The Oort cloud is separate from the Trans-Neptune region, and much farther out.

Plane of the ecliptic

The plane of the ecliptic is defined by the Earth's orbit around the Sun. All of the planets orbit the Sun roughly around this same plane. The farther away from this plane a planet orbits, the more inclined is its orbit to the ecliptic. If you could look at the solar system "edge on" then all the planets would be orbiting in the plane of the ecliptic plane.

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